Tweaking the Asian soybean rust alarm

A few months ago, when reports of overwintering Asian soybean rust began filtering out of Mexico, Mississippi Extension employees knew their battle plan needed a quick tweaking.

“We’ll be locating sentinel plots a little differently,” says Billy Moore, Mississippi Extension plant pathologist. “Across the southern part of the state the plots will remain extremely important. But now that we’ve got rust in northeast Mexico and Texas, it makes it equally important to watch the Delta side of the state. So we’ve got sentinel plots planted from about 12 miles north of Clarksdale to Natchez and across to Jackson County and north to Lee County.”

Originally, Moore and colleagues planned for 15 plots but soon discovered they needed more to provide producers a proper alarm system. Currently, 16 of the planned 18 sentinel plots have been planted.

Producer sentinel fields

In addition to the sentinel plots, Mississippi’s ASR team will also be monitoring early-planted producer fields. That’s particularly true in the western part of the state where there are larger gaps between sentinel plots.

How were participating producers located?

“We knew of these early-planting producers,” says Moore. “In fact, one producer in particular will be extremely important in this effort. He’s north of the sentinel plot at the Delta Station (in Stoneville) and south of Coahoma County, where there’s another plot. We felt there was too big a space between those plots and needed to have a willing, good producer who would allow us to monitor his fields.”

This year, Mississippi’s sentinel plots are composed of Group 4s and Group 5s (a “mid” and “late” of each). In addition, researchers will also be monitoring 26 SMART (Soybean Management by Application of Research and Technology — the state’s soybean verification program) fields. The sentinel plots and fields will be monitored weekly until bloom and, after that, every two weeks.

“The sentinel plot effort worked beautifully for us last year. The first two locations we found ASR in were in sentinel plots. The same was true in Alabama and the Florida panhandle.

“That doesn’t mean the same scenario will repeat this season. But we hope we have an advantage by having it attack sentinel plots before producer fields. That would give us an alert to call for spray applications, if needed.”

Stages and Brazil

All stages of the soybean crop are susceptible to ASR. Moore cautions that last year Brazil had optimal conditions in the Matto Grasso area and ASR occurred on vegetative soybeans.

However, the most susceptible stages to ASR are reproductive — flowering and later. Moore hopes the sentinel plots reach the most susceptible stages well in advance of the bulk of producers’ crops.

“If ASR does move in, we’ll be able to detect it on sentinel plot beans that are blooming or setting pods while producers’ crops haven’t yet reached that stage.”

There are differences between Mississippi and Matto Grasso soybeans and ASR.

“Brazil has a heavy innoculum down there year-round. Consequently, even in the vegetative stage, their innoculum load is tremendously higher than what ours would be. That’s because for Delta state beans, the disease must move in from areas where it overwintered: Mexico, southern Florida, the Caribbean, potentially. So, the hope is that the United State will start each growing season with a low innoculum level.”

That points away from a major ASR problem during the Mississippi crops’ vegetative stage. If it occurs then, it’ll likely be very light — “perhaps so light we won’t be able to detect it. And if we do detect it, if at all possible, we’ll wait until the reproductive stages to treat. Producers can’t afford to treat with fungicides too many times.”

Plugging loopholes

Despite everyone’s best efforts, “there are still some loopholes we’re trying address.” For example: no ASR occurs early and sentinel plots are at R-5. Meanwhile, the bulk of the producers’ crop is at R-1 to R-4, making all the plants fairly equal in terms of susceptibility. That means rust has just as good a chance of occurring on a producer’s crop as on a sentinel plot.

“How can we maintain an ‘alarm system’ in such a scenario? That’s something that needs to be plugged. There’s never a case in agriculture where risk can be completely eliminated. But we’re trying.”

One thing being studied is the efficacy of a spore “wet” trap. If they work, the funnel-shaped traps will catch rainfall. Any spores in the raindrops will stick to filter paper inside the traps. The paper will be collected regularly and examined for spores.

“The idea of the traps is to let us know when spores wash out of the atmosphere into fields. Hopefully, any spores caught will be of enough quality and numbers to run lab tests to rule ASR in or out. If successful, that could be a trigger for fungicide applications.”

Kudzu questions

Home to some 250,000 acres of the consuming vine, Mississippi has the largest kudzu acreage in the nation.

“We’ve got a bunch to keep an eye on,” says Moore. “In mid-March, we began seeing new kudzu leaves developing. To make sure Asian soybean rust (ASR) isn’t occurring on it, we’ll be looking at kudzu in the southern part of the state very closely at least through early April.”

While it’s common knowledge that kudzu is a host for ASR, Moore says last year’s events are puzzling.

“Kudzu doesn’t appear to be as susceptible to rust as soybeans. Last year, the first ASR find in Mississippi was in a sentinel plot on the George County/Jackson County line.”

Less than a mile from the sentinel plot were three soybean fields.

“We suggested those fields be sprayed (with a fungicide) at R-3/R-4 and (the producer) did. That one application provided great control of rust for 21 to 28 days. After that, rust came in and it was extremely heavy in those fields just prior to defoliation. There were often hundreds of pustules per leaf.”

Moore says between the aforementioned sentinel plot and soybean fields is a “massive” area of kudzu. Located about a quarter mile from the beans, the kudzu “chokes off at least 100 yards. It’s in the trees and covers everything.

“Until frost, we looked hard to find pustules in that kudzu. We checked protected areas, the vines in the trees, everywhere. We couldn’t find a single pustule. So why didn’t ASR occur on the kudzu?”

To Moore, it appears kudzu simply isn’t as susceptible to ASR as soybeans.

“And I’m not the only one wondering if that’s a possibility. Prior to frost killing the kudzu last fall, several researchers…asked me to go through Mississippi and collect kudzu seed. Those will be used in tests to determine if there is a difference in susceptibility. Seed was collected from 50 sites around the state and I’m very curious to see what will be found. It’s possible the studies will show there are susceptibility differences between kudzu patches.”

Windshield scouting

If conditions are optimal, a spore that lands in a soybean field takes 1.5 hours to germinate into fungus and six hours to penetrate the plant.

“From the time the fungus enters the leaf to the point symptoms are seen, seven or eight days can pass. That’s a long time for a fungus to be developing and a producer not be aware of it. That means the quicker we’re able to spot it, the better off we’ll be. That points to everyone not being content to check the crop through the truck windshield. Producers need to walk their fields.”

Hopefully, sentinel plots will work as efficiently this year as they did last. But Moore says there is a possibility of a temperature inversion in Florida that could carry spores high into the atmosphere.

While ASR spores are sensitive to solar radiation, “all it takes is a few survivors. In such a case, the spores could fly over our sentinel plots and land in, say, northern Mississippi. That could be the first place rust is found. That possibility is slim, but producers, despite early-warning efforts, need to be looking for anything unusual in their fields.”

Based on what he saw last year, Moore is impressed with how fungicides control the disease. “Even what I’d consider the weakest fungicide performed tremendously well in south Alabama in a 60-acre field. With timely, good coverage, fungicides will work for producers.